drm/i915/gtt: Revert "Disable read-only support under GVT"

This reverts commit c9e66688.

Checked GVT codes that guest PPGTT PTE flag bits are propagated
to shadow PTE. Read/write bit is not changed. Further tested by
i915 self-test case "igt_ctx_readonly". No error or GPU hang was
detected. So enable read-only support under GVT.

Signed-off-by: Hang Yuan <hang.yuan@linux.intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1540883281-11359-1-git-send-email-hang.yuan@linux.intel.com


Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>